Going to play it safe and say either CYMA AK or G&G CM16. G&Gs are great starter guns and decently cheap, and practically everywhere sells them. You also have many options with individual guns - from Firehawk (tiny but loud) to Raider (standard M4) to R8-L (basically a DMR).

CYMA's AKs are normally really good, I used to have their AK-74M. Their AKs normally range from £130 to about £200. Personally I'd recommend their AKS-74u, it's short and great for all environments as well as being full metal. Forcecore AKs (made by E&L) are also a pretty good option.

Great coincidence there then! You could be okay but personally I'd go with the CYMA just due to being familiar with it. In terms of mags, pretty much any version of AK mag will fit - in my 74M (the AK-74M and AKS-74u have the same receiver), I could fit CYMA AK-74 mags, CYMA AK-47 mags and a TM Beta Spetsnaz mag. All fed perfectly. Where would you put the rail?

It's really up to you. As someone who's owned both an AK and an M4, I preferred the AK just because of how different it was. The M4 is more versatile and has slightly better ergonomics, whereas the AK seems generally cooler and has a slightly better gearbox design (V3 rather than V2). Whatever's best for you, at the end of the day!